The present invention relates to a device for distributing a microwave signal between the radiating elements of an array antenna.
For feeding array antennas with frequencies within the microwave range, different networks usually, for example, make use of stripline technology or waveguides. The requirements of the networks are to give a constant feed to the radiating elements of the antenna within the used frequency band, both with regard to amplitude as well as to phase. This is important to insure that the desired radiating characteristics are obtained. Particularly low sidelobe levels put high demands on the accuracy of the feed. Additional demands on the network are to manage occurring power levels and to allow a sufficiently compact placement of the outputs of the network, which is determined by the separation of the radiating elements which is usually of the order of 0.5-0.7 wavelengths.
A complicating factor in this context is that the radiating elements show a varying impedance when the frequency and radiating direction are changed. The latter can for example be controlled by a phase changer. In cases like the present one it is usual to speak of the "active impedance" of the elements which consequently change during operation. In spite of this, it is required that the feed of the elements can be done so that the excitation becomes the intended one (prescribed amplitude, usually linearly changing phase) in spite of the mentioned load variations.
A common type of antenna has vertical electrical lobe control, but a sideways fixed lobe. Such an antenna has two sets of feed networks, a plurality (often alike) for the feeding of every horizontal row of the antenna, as well as one with built in variable phase changers that feed the individual rows vertically. It is especially important in these cases to obtain low weight and low manufacturing costs for the fixed horizontal networks, as these occur in a great number in each antenna.
Such compact feeding networks are feasible in stripline technology. This, however, gives several disadvantages, such as high losses and poor power sustainability. A better technology from many points of view is to use feeding networks realized with waveguides.
In order to i.a. be able to attain a satisfactory bandwidth, it is essential that the electrical length from the feeding point of the antenna to each radiating element is the same. This can easily be attained with a waveguide network that is constructed as repeating parallel junctions. Such a network does however acquire large dimensions and an extreme weight, which often cannot be accepted.
Another waveguide solution can be based on serial feeding, which gives smaller dimensions, but usually an unwanted frequency-dependent lobe direction.
To be able to cope with the load variations from the radiating elements, it might be necessary to use branching components (power divider) of the four port type. The fourth port is terminated and used for absorbing possible imbalances of the reflections from the load. Possible components are the magic T, 90.degree. hybrids etc. These are however mostly all too bulky, and they also increase the costs.
Different serial feed array antennas are known. The American patent U.S. Pat. No. 3,438,040 is an example of a device where the radiating elements of an array antenna are serially fed. The power division would seem to be done by means of variation of the waveguide dimensions. This solution of the problem is however less suitable since the power division should be done in the magnetic plane, because a change of the waveguide width will influence both the waveguide wavelength and as well as the impedance.
The American patent U.S. Pat. No. 3,977,006 also describes a serially fed array antenna. In this, the power is distributed by means of slots in a feed waveguide, whereby each slot feeds a waveguide connected to a radiating element. Due to the polarization rotation in the slots, the fed waveguides have to be placed 90.degree. rotated in relation to the feeding waveguide, an arrangement that becomes bulky, especially "vertically". Because the characteristics of the slots are frequency dependent, the device will furthermore have a proportionately narrow bandwidth.